The purpose of this study was to investigate kinematic characteristics of the overarm throwing motion for elementary school boys by using three-dimensional motion analysis techniques.
Thirty-four boys (12 for the 1st grade, 11 for the 3rd grade, and 11 for the 5th grade) were selected as subjects. Their overarm throwing motions were videotaped with two high-speed VTR cameras. Three-dimensional coordinates of body segment endpoints and the ball were obtained by a direct linear transformation (DLT) method. Kinematic parameters computed were: release velocity and angle of the ball, step length, backward/forward lean and right/left flexion angles of the trunk, abduction/adduction, horizontal abduction/adduction and internal/external rotation angles of the shoulder joint, flexion/extension angles of the elbow joint for the throwing arm, flexion/extension angles of the knee joint, abduction/adduction and flexion/extension angles of the right hip joint, and rotation angles of shoulders and hips. Throwing motion phase was divided into two phases: the first phase was from the maximal back swing of the throwing arm (MBS) to step foot contact (F-ON), and the second phase was from F-ON to the ball release (REL). Kinematic data were normalized by these phase times and averaged to obtain mean patterns.
Throwing distance and release velocity of the ball significantly increased with the increase in school grades. Step length tended to increase with the increase in school grades, and significant difference in step length was found between the 1st grade and the 3rd grade. Step-length/body-height significantly increased from the 1st grade to the 3rd grade, but not to the 5th grade.
Motions of the lower limbs, trunk and upper limbs changed with the school grades. In the 3rd grade, the flexion and the abduction angles of the right hip joint and the flexion angle of the right knee joint significantly increased in the first phase, and the backward lean of the trunk appeared in the second phase. In the 5th grade, the trunk showed the maximal right flexion at the first half of the first phase, and decreased the right flexion before F-ON. And the forward lean of the trunk in the first phase changed to the backward lean before F-ON. The external rotation angle of the shoulder joint at REL was significantly smaller in the 3rd grade than in the 1st grade. In the 5th grade, the horizontal abduction of the shoulder joint appeared in the first phase.
